Diesel engines have been employed as engines for over-the-road vehicles because of relatively low fuel costs and improved mileage. However, because of their operating characteristics, diesel engines discharge a larger amount of carbon black particles or very fine condensate particles or agglomerates thereof as compared to the gasoline engine. These particles or condensates are sometimes referred to as "diesel soot", and the emission of such particles or soot results in pollution and is undesirable. Moreover, diesel soot has been observed to be rich in condensed, polynuclear hydrocarbons, and some of these have been recognized as carcinogenic. Accordingly, particulate traps or filters have been designed for use with diesel engines that are capable of collecting carbon black and condensate particles.
Conventionally, the particulate traps or filters have been composed of a heat-resistant filter element which is formed of porous ceramic or metal fiber and an electric heater for heating and igniting carbon particulates collected by the filter element. The heater is required because the temperatures of the diesel exhaust gas under normal operating conditions are insufficient to burn off the accumulated soot collected in the filter or trap. Generally, temperatures of about 450.degree.-600.degree. C. are required, and the heater provides the necessary increase of the exhaust temperature in order to ignite the particles collected in the trap and to regenerate the trap. Otherwise, there is an accumulation of carbon black, and the trap is eventually plugged causing operational problems due to exhaust back pressure buildup. The above-described heated traps do not provide a complete solution to the problem because the temperature of the exhaust gases is lower than the ignition temperature of carbon particulates while the vehicle runs under normal conditions, and the heat generated by the electric heater is withdrawn by the flowing exhaust gases when the volume of flowing exhaust gases is large. Alternatively, higher temperatures in the trap can be achieved by periodically enriching the air/fuel mixture burned in the diesel engine thereby producing a higher exhaust gas temperature. However, higher temperatures can cause run-away regeneration leading to high localized temperatures which can damage the trap.
It also has been suggested that the particle build-up in the traps can be controlled by lowering the ignition temperature of the particulates so that the particles begin burning at the lowest possible temperatures. One method of lowering the ignition temperature involves the addition of a combustion improver to the exhaust particulate, and the most practical way to effect the addition of the combustion improver to the exhaust particulate is by adding the combustion improver to the fuel. Copper compounds have been suggested as combustion improvers for fuels including diesel fuels.
The U.S. Environmental Protection Agency (EPA) estimates that the average sulfur content of on-highway diesel fuel is approximately 0.25% by weight and has required this level be reduced to no more than 0.05% by weight by Oct. 1, 1993. The EPA has also required that this diesel fuel have a minimum cetane index specification of 40 (or meet a maximum aromatics level of 35%). The objective of this rule is to reduce sulfate particulate and carbonaceous and organic particulate emissions. See, Federal Register, Vol. 55, No. 162, Aug. 21, 1990, pp. 34120-34151. Low-sulfur diesel fuels and technology for meeting these emission requirements have not yet been commercially implemented. One approach to meeting these requirements is to provide a low-sulfur diesel fuel additive that can be effectively used in a low-sulfur diesel fuel environment to reduce the ignition temperatures of soot that is collected in the particulate traps of diesel engines.
U.S. Pat. No. 3,346,493 discloses lubricating compositions containing metal complexes made of the reaction products of hydrocarbon-substituted succinic acid (e.g., polyisobutylene-substituted succinic anhydride) compounds and alkylene amines (e.g., polyalkylene polyamines), the complexes being formed by reacting at least about 0.1 equivalent of a complex-forming metal compound with the reaction products. The metals are those having atomic numbers from 24 to 30 (i.e., Cr, Mn, Fe, Co, Ni, Cu and Zn).
U.S. Pat. No. 4,673,412 discloses fuel compositions (e.g., diesel fuels, distillate fuels, heating oils, residual fuels, bunker fuels) containing a metal compound and an oxime. The reference indicates that fuels containing this combination are stable upon storage and effective in reducing soot formation in the exhaust gas of an internal combustion engine. A preferred metal compound is a transition metal complex of a Mannich base, the Mannich base being derived from (A) an aromatic phenol, (B) an aldehyde or a ketone, and (C) a hydroxyl- and/or thiol-containing amine. Desirable metals are identified as being Cu, Fe, Zn, Co, Ni and Mn.
U.S. Pat. No. 4,816,038 discloses fuel compositions (e.g., diesel fuels, distillate fuels, heating oils, residual fuels, bunker fuels) containing the reaction product of a transition metal complex of a hydroxyl- and/or thiol-containing aromatic Mannich with a Schiff base. The reference indicates that fuels containing this combination are stable upon storage and effective in reducing soot formation in the exhaust gas of an internal combustion engine. The Mannich is derived from (A) a hydroxyl- and/or thiol-containing aromatic, (B) an aldehyde or a ketone, and (C) a hydroxyl- and/or thiol-containing amine. Desirable metals are identified as being Cu, Fe, Zn and Mn.
International Publication No. WO 88/02392 discloses a method for operating a diesel engine equipped with an exhaust system particulate trap to reduce the build-up of exhaust particles collected in the trap. The method comprises operating the diesel engine with a fuel containing an effective amount of a titanium or zirconium compound or complex to lower the ignition temperature of the exhaust particulates collected in the trap.